1. Field of the Invention
The present invention relates to recalling a volume in a virtual tape server, and more particularly to a technique for optimizing the processing order of volume recall requests from the viewpoint of minimizing the average latency time of a host apparatus issuing the requests.
2. Description of the Related Art
A virtual tape storage system emulates a tape drive and a tape cartridge by using a hard disk drive. For example, the host apparatus performs the input/output (I/O) operation for a tape library by performing the I/O operation for one set of hard disk drives emulating the tape library. In the virtual tape storage system of the prior art, such as a Magstar virtual tape server of IBM® (International Business Machines (IBM® is a registered trademark of IBM Corporation of Armonk, N.Y.)), at least one virtual tape server (VTS) is connected to the tape library including a plurality of tape drives and the tape cartridge. The VTS is also connected to a DASD (Direct Access Storage Device) composed of a plurality of hard disk drives interconnected.
The DASD functions as a cache for the volume within the tape library. If receiving a request from the host apparatus, the VTS reads the requested volume from the cache and returns it to the host apparatus. If the requested volume does not exist in the cache, the VTS recalls the volume from the tape library to the cache. The cache can satisfy the I/O request more quickly than the tape library. However, since the capacity of cache is relatively smaller than that of tape library, only the volume to be accessed frequently is held in the cache. The VTS transfers the less frequently accessed volume from the cache to the tape library, using an algorithm such as LRU (Least Recently Used). The transferred volume is removed from the cache and abbreviated to a pointer pointing to the data on the tape cartridge, as one example.
In the typical VTS, the number of virtual devices providing access to the virtual volume is larger than the number of tape drives usable for recalling the volume. Therefore, more virtual devices than the tape drives may gain access to the virtual volume that does not exist in the cache. In such a case, a recall issued later is put into the queue for process limited. Generally, these recalls put into the queue are processed in first-in first-out order. A problem here is the time taken to recall.
For example, when the tape cartridge containing the recalled data is not mounted on any tape drive, one of the tape cartridges mounted at present must be unloaded from the tape drive. This process may possibly take one minute or more. Then, the tape cartridge containing the target data is loaded into the tape drive, and the tape is positioned at the top of data. This process also may possibly take one minute or more. Further, when all the tape drives are in use, it is required to wait until the process is ended in any tape drive. The time for waiting until the process is ended depends on the time required to perform the active process.
As the prior art for solving this problem and increasing the efficiency of recall request, there is Japanese Patent Application No. 2006-190274. Japanese Patent Application No. 2006-190274 discloses a technique for optimizing the queuing process by rearranging the queue in accordance with one or more optimization rules. As the optimization rules, Japanese Patent Application No. 2006-190274 discloses grouping the minimum number or maximum number of recall requests corresponding to the logical volume resident on a single mountable medium within the queue successively, or complying with the physical locations of at least two recall requests on the mountable medium in grouping. Japanese Patent Application No. 2006-190274 also discloses a technique for processing the first recall request based on a situation of the second recall request, or processing a specific recall request by ignoring the optimization rules if the minimum latency threshold value is reached, thereby preventing the requester from waiting for processing of any specific request too long.
With the technique as disclosed in Japanese Patent Application No. 2006-190274, the efficiency of recall request can be increased. However, this is the increased efficiency of recall request as seen from the VTS, but not from the host apparatus. The technique of Japanese Patent Application No. 2006-190274 makes the time for the VTS to use the tape drive as short as possible. However, the latency time of the host apparatus is the time since the VTS accepts the request of the host apparatus until the process is completed, and greatly depends on the time taken to process the request accepted previously by the VTS.
For example, if a request requiring a very long time to process is accepted previously by the VTS, a request issued immediately after it and put in the queue must wait for a long processing time of the previous request, though it can be intrinsically processed in a short time. As a result, the average latency time of the host apparatus is longer. In Japanese Patent Application No. 2006-190274, it is considered to assure the response time to the host apparatus by providing the minimum latency threshold value. However, with the technique as disclosed in Japanese Patent Application No. 2006-190274, the average latency time of the host apparatus is not shortened, though the requester is prevented from waiting for processing of any specific request too long. Actually, the actual processing time for each request is not considered in Japanese Patent Application No. 2006-190274.